Typically, communication systems such as mobile telephones or cellular radio telephone systems require frequency band-pass filters having frequency ranges from tens of Megahertz to Gigahertz, and having fractional bandwidths covering a range from less than 0.01% to about 5.0%. The foregoing frequency ranges and bandwidths are suitable for both centre frequency and bandwidth for the intermediate frequency (IF) and radio frequency (RF) stages of such communication systems.
Well-known frequency filtering techniques for these frequency ranges are lumped LC filters, spiral or helical filters, dielectric filters and bulk acoustic wave filters. In addition to the foregoing there are also known electro-acoustic devices such as SAW coupled resonator filters, which have found particular applications in mobile communication systems since they are generally of lower volume and size, have better electrical performances, manufacturing uniformity and stability. Additionally, SAW devices can be manufactured to a substantially consistent standard, and hence there is less need to tune devices after fabrication. Comparison of coupled resonator filter techniques to other types of SAW band-pass filter techniques shows that the coupled resonator filter has a relatively low loss over its operating bandwidth with a high out of band rejection and small transition bandwidth.
Due to the low loss, high out of band rejection and small transition bandwidths typically associated with SAW coupled resonator filters including acoustic transversely coupled resonator filters and acoustic in-line coupled resonator filters, SAW coupled resonator filters are found to be particularly suitable for frequency band-pass filtering from narrow to moderate bandwidths. They are particularly suitable for miniaturised analogue and digital mobile communication systems as RF and IF filters.
FIG. 1 shows a schematic diagram of a conventional 2-pole transversely coupled resonator filter (TCF) 100. The TCF is composed of two acoustic tracks 102, 104 which are disposed adjacent to each other. Each track consists of one inter-digital transducer (IDT) 106, 108 and two reflection gratings 110, 112, 114, 116 symmetrically disposed at each side of respective IDTs 106, 108. Each IDT 106, 108 comprises an array of transducer electrodes 120, 122 arranged in a comb-like fashion and interleaved with each other. A common bus bar 118 is coupled to the transducer electrodes on adjacent sides of the respective IDTs 106, 108. A common bus bar is advantageous in that adjacent acoustic tracks 102, 104 can be disposed closer together than if separate bus bars were used, which results in a stronger acoustically transverse coupling between adjacent tracks than for separate bus bars. Typically, the common bus bar 118 extends to the reflection gratings which in the example shown in FIG. 1 comprise earthed electrodes 124. When one or other of the IDTs 106, 108 is electrically excited an acoustic wave is excited within the respective acoustic track 102, 104. Due to the proximity of the acoustic tracks, an acoustic wave which is guided in a first acoustic track, 102 for example, is coupled into the second acoustic track 104 by virtue of an overlap of the guided wave profile tail in the first acoustic track, thereby exciting an acoustic wave in the second acoustic track. The acoustic wave in the second acoustic track then generates an appropriate electric signal in the IDT, 108 in this example, for the second acoustic track, and an electrical signal is output from the filter. The spaces between the IDT 106, 108 and each reflection grating can be covered with a conductive film or may be a free surface. Further details of transversely coupled resonator filters may be found in European Patent Application EP 0 100 503.
FIG. 2 shows a typical in-line coupled resonator filter 200. The in-line coupled resonator filter shown in FIG. 2 consists of three IDTs 202, 204 and 206 and two reflection gratings 210 and 212 collinearly positioned with respect to each other. The two outer IDTs 202 and 204 are parallel-connected and IDT 206 is disposed between them. The whole system shown in FIG. 2, including the reflection gratings 210 and 212 comprises a cavity resonant system having two electric ports. A first electric port is formed by IDTs 202 and 204 and a second electric port is formed by IDT 206. As discussed with reference to FIG. 1 each of the IDTs 202, 204, 206 consist of arrays of interleaved transducer electrodes 208. The reflection gratings comprise earthed electrodes 214. In operation, when one electric port of the in-line coupled resonator filter is driven by an applied voltage acoustic cavity modes are excited within the resonant system and the other electric port couples to these acoustic modes to produce an electric output signal.
Both the conventional coupled resonator filters described above are suitable for only unbalanced driving and loading at their input and output ports. Thus, they are only capable of being directly coupled to devices having unbalanced inputs or outputs. Thus, they are not particularly suitable for a number of applications, for example, an IF band-pass filter coupled to a balanced mixer. For applications where the conventional coupled resonator filter is to be coupled to balanced inputs or outputs an appropriate balanced-unbalanced (BALUN) transition is required. Such transitions are typically lossy and furthermore take up space either on a circuit board upon which they are typically etched or by virtue of the lumped element components comprising the BALUN. Additionally, when more than one coupled resonator filters are cascaded together to form multi-track, multi-pole filters, the necessary ground connections between respective tracks of the cascaded filters result in cross-talk which seriously degrades the performance of such multi-pole filters. Such degradation in performance is particularly noticeable in the out of band regions of such filters. Furthermore, having to provide ground connections between tracks of multi-track devices makes the layout of the device more complex. In many cases the connections can only be performed by using bonding wires. Such bonding wires or flying leads introduce parasitic electrical components into the electrical characteristics of the device and thereby further degrade the device's performance.